siddarth chandrasekaran “advanced spectroscopy in chemistry” “advanced spectroscopy in...
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Siddarth ChandrasekaranSiddarth Chandrasekaran “ “Advanced Spectroscopy in Chemistry”Advanced Spectroscopy in Chemistry”
University of LeipzigUniversity of Leipzig18/12/200918/12/2009
Module: Spectroscopy of Fluid Interfaces (13-122-0412)
IndexIndex
Understanding MIES spectra
Data Analysis Linear Combination Singular Value Decomposition
Applications of Data Analysis
Conclusion
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Understanding MIES spectra Understanding MIES spectra
Max. B.E. depends on source He 23S – 19.8 eV He 21S – 20.6 eV
Low penetration, outermost orbitals interact
Information about spin-orbit coupling, too
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Kim et al, J. Phys. Chem. B 107, (2003), 592-596
Understanding MIES spectraUnderstanding MIES spectra
Chemical shift can be observed
For example: lowering of Binding Energy, because of neighbors
Useful for characterizing surface reactions
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Kim et al, J. Phys. Chem. B 107, (2003), 592-596
Chemical ShiftChemical Shift
Sum of work function of surface and Binding energy of 5p1/2 for adsorbed Xe constant
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Kim et al, J. Phys. Chem. B 107, (2003), 592-596
Data AnalysisData Analysis
What Data? MIES spectra Important Prerequisite: Good spectra, so try to
record best possible spectra
Why Analysis? Improve quality of data
varies from simple baseline corrections to complicated mathematical calculations
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Data analysisData analysis
Helps to extract hidden (latent) information, but cannot create information
Multicomponent mixtures - Fraction of species present on the surface – QUANTITATIVE Analysis
In this talk focus is on Linear Combination method and Singular Value Decomposition (SVD)
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Linear Combination MethodLinear Combination Method
When liquids with similar surface tensions are mixed Smixture = a1Sspecies,1+a2Sspecies,2+….+anSspecies,n
S - spectra a – surface fraction of the species
Only possible in the case of physical homogeneous (macroscopically homogeneous) mixtures No orientational effects No large domain formations
We need to know the pure spectra of the components
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Linear combination MethodLinear combination Method
Reference Spectra
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H. Morgner* & M. Wulf , J. of Elec. Spec. and Rel. Phen. 74 (1995) 91-97
Linear Combination MethodLinear Combination Method
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H. Morgner et aI. , Molecular Physics, 73, (1991), No. 6, 1295-1306
Smix = aBA* SBA + aFA* SFA
aBA + aFA = 1
Inference: Linear combination of spectra are very effective in a few simple cases
Example where linear combination not Example where linear combination not possiblepossible
The reaction has at least two intermediates with variable conc.'s which couldn’t be identified in this paper
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Lescop et al, Surface Science 565, (2004), 223-231
Why Singular Value Decomposition Why Singular Value Decomposition (SVD)(SVD)
When linear combination of individual spectra not enough to reproduce the total spectra
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When & what SVD?When & what SVD?
What information can we get from SVD No. of components & their compositions Spectra of unknown components possible
Pure spectra of one species can be obtained from mixture of species, especially useful when Single monolayer spectra cannot be recorded Orientational effects or chemical reactions
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Singular Value Decomposition (SVD)Singular Value Decomposition (SVD)
Handy mathematical technique that has application to many problems
Given any mn matrix A, algorithm to find matrices U, V, and W such that
A = U W VT
U is mn and orthonormal
W is nn and diagonal
V is nn and orthonormal
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SVDSVD
code used in Matlab [U,W,V]=svd(A,0);
Matrix A contains the spectra recorded
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SVD on 27 different spectraSVD on 27 different spectra(optical spectroscopy)(optical spectroscopy)
SVD to be performed on the above spectra
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Performed SVD to get U,W & V matrix
W- MatrixW- Matrix
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The W-Matrix obtained by using the SVD algorithm
The diagonal elements in percentage values to highlight the importance of the value
Choice of no. of componentsChoice of no. of components
Red and Green line overlaps almost perfectly
Two components not enough to reproduce spectra
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U- Matrix for first three componentsU- Matrix for first three components
The columns of the U-matrix have no physical significance. Negative peaks
Linear combinations of the elements of the U-Matrix can represent spectra
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Obtaining spectra of unknown Obtaining spectra of unknown componentscomponents
Lets consider three species system
Smixture = aαSspecies α+aβSspecies β+aγSspeciesγ
aα+ aβ + aγ = 1
In ideal case we know Sspecies α & Sspecies β
Sspecies γ = a1B1 + a2B2 + a3B3
B1, B2, & B3 are basis of the U matix
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PROBLEM : Pure spectra of solute (e.g.: salt) cannot be observed in liquid state
Earlier Methods used Difference spectra Ssalt = S salt+solvent – a * Ssolvent
S is spectra & a is scaling factor (both are input parameters)
Peak areas fitting by ratio of salt/solvent Intrinsic knowledge of intensity, position and linewidth of
solvent spectra Lots of assumptions
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Determination of pure spectra of TBAIDetermination of pure spectra of TBAI
J. Oberbrodhage*,J. of Elec. Spec. and Rel. Phen.107 (2000) 231–238
Determination of pure spectra of TBAIDetermination of pure spectra of TBAI
MIE reference data of the pure solvents formamide and hydroxy-propionitrile.
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J. Oberbrodhage*,J. of Elec. Spec. and Rel. Phen.107 (2000) 231–238
Three base spectra sufficient
We expect three species – FA, TBAI & HPN
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Determination of pure spectra of TBAIDetermination of pure spectra of TBAI
J. Oberbrodhage*,J. of Elec. Spec. and Rel. Phen.107 (2000) 231–238
Results obtained by SVD comparable with that by difference spectra method
Greater sensitivity because of lower noise
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Determination of pure spectra of TBAIDetermination of pure spectra of TBAI
J. Oberbrodhage*,J. of Elec. Spec. and Rel. Phen.107 (2000) 231–238
MIES used to evaluate the surface fraction of each of the species
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Determination of pure spectraDetermination of pure spectra
J. Oberbrodhage*,J. of Elec. Spec. and Rel. Phen.107 (2000) 231–238
Determination of spectra of unknown Determination of spectra of unknown component component
Mixture of Pentadecane (PD) and Formamide (FA)
The linear combination using only two species was not enough and hence need for third component
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H. Morgner*, J. Oberbrodhage, J. of Elec. Spec. and Rel. Phen. 87 (1997) 9-18
Third component spectra similar to that of a standing alkane – orientation of the alkane (PD) can be seen
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Determination of spectra of unknown Determination of spectra of unknown component component
H. Morgner*, J. Oberbrodhage, J. of Elec. Spec. and Rel. Phen. 87 (1997) 9-18
Percentage contribution of each species is shown in the graph to the left
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Determination of spectra of unknown Determination of spectra of unknown component component
H. Morgner*, J. Oberbrodhage, J. of Elec. Spec. and Rel. Phen. 87 (1997) 9-18
ConclusionConclusion
MIES – Surface specific
Data Analysis techniques like SVD & Linear Combinations are tools to extract hidden information
SVD is rather simple when we have acquired good quality spectra But there is a need for good computational
abilities and high speed computers
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THANK YOU for your THANK YOU for your attentionattention
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Metastables Electron Emission Microscopy (MEEM)MEEM)
Controlling Helium beam diameter difficult
Area from which electrons are abstracted can be controlled – spatial resolution
Surface electron can be mapped non-destructively
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Harada et al*, Nature 372 (1994) 657-659